1. Field of the Invention
The present invention relates broadly to the treatment and recycle of rubber products and still more particularly to the use of recycled crumb rubber. The United States Government has rights to this invention pursuant to contract number DE-AC09-96-SR18500 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Existing efforts to recycle used rubber, in particular used tires, into new rubber articles, especially tires, have met with only limited commercial success. In the United States alone, there are currently billions of tires stock-piled in long term storage with additional millions being added annually to such stock piles. Because of the large volume involved with tires, this discussion will be directed to tires although many of these comments are applicable to other new and used rubber products.
Because used rubber is usually processed in the form of crumb rubber, references herein will be to rubber in that form. However, crumb rubber is merely one example of a used rubber product suitable for processing as described herein. An existing limitation in the recycling of used tire material is that the used tire rubber cannot be readily mixed in economical proportions to form suitable new tire polymer mixes having acceptable cured properties.
During the vulcanization process of new tires, chemical accelerators, promoters, and/or initiators, are used and large numbers of sulfur-sulfur and sulfur-carbon cross-links are produced in the vulcanized tire rubber. It is generally believed in the art that the sulfur compounds which are present in used tire rubber are detrimental in the curing process when used tire material is mixed with new tire polymer. Formulations having significant levels of used tire rubber particles with new polymer materials result in a brittle compound unsuitable for many uses such as automobile or truck tires. Heretofore, efforts to reclaim scrap rubber have frequently included a physical sheering process which is suitable for a rubber which can be mixed with asphalt, forming asphalt rubber. Such use is taught in U.S. Pat. No. 5,304,576.
It is also known to take used rubber and depolymerize the vulcanized rubber in an organic solvent and then recover various polymerized fractions as taught in U.S. Pat. No. 5,438,078. Similarly, U.S. Pat. No. 5,264,640 teaches taking scrap rubber from used tires and regenerating the monomeric chemicals which are subsequently recovered. This method uses gaseous ozone to break down the crosslinked structure of the rubber followed by thermal depolymerization in a reaction chamber. U.S. Pat. No. 5,369,215 teaches a similar process in which used tire material may be depolymerized under elevated temperatures and at a reduced pressure to recover the monomeric compounds.
U.S. Pat. No. 4,104,205 teaches a microwave method to devulcanize rubber from hose end trim and butyl tire bladders. While tire tread material was also treated, difficulties in exothermic reactions and physical properties of the microwaved materials were noted.
U.S. Pat. No. 4,341,667 teaches that the green strength of elastomers reclaimed through heat, microwave, chemical treatments, or physical shearing can be improved by the addition of butene polymers.
U.S. Pat. No. 5,275,948, incorporated herein by reference, teaches the use of chemolithotrophic microorganisms to remove sulfur from the surface of finely ground scrap rubber. Preferred organisms include Thiobacillus species which oxidize elemental sulfur to sulfuric acid and which are released into the suspension culture.
U.S. Pat. No. 5,597,851, incorporated herein by reference, teaches the use of microorganisms to desulfurize finely ground rubber particles. Thiobacillus sp. and Sulfolobus acidocaldarius are used to bioprocess rubber particles for at least 24 hours but prior to complete oxidation of surface sulfur.
The present application relates to commonly assigned U.S. Application having Ser. No. 09/542,394, filed Apr. 4, 2000, entitled xe2x80x9cCombination Biological and Microwave Treatments of Used Rubberxe2x80x9d and having Attorney Docket No. WSR-14, and which is incorporated herein by reference. The present application also relates to commonly assigned U.S. patent application Ser. No. 09/542,201, filed Apr. 4, 2000, entitled xe2x80x9cMicrowave Treatment of Vulcanized Rubberxe2x80x9d, and which is incorporated herein by reference.
There remains a strong need for a practical, economical system for processing used rubber into a material which can be incorporated at a substantial loading level into new rubber compounding mixtures. Further, there is room for improvement with respect to bioprocessing of used rubber, particularly in the selection of microorganisms, the selection of starting material, and overall process conditions and parameters.
The present invention is a process and the resulting product of the process in which previously vulcanized (used) rubber may be incorporated into polymer mixes for new rubber products, including tires, at much greater levels than used heretofore.
The present invention also provides a process and a resulting product of the process wherein previously vulcanized, used crumb rubber has the surface chemistry altered to provide a treated crumb rubber product. When compared to untreated crumb rubber, the treated crumb rubber has generally improved properties useful for incorporation with new tire or virgin rubber into automobile and truck tire polymer formulations. The improved properties for the composite polymer formulation include plasticity, elongation @ break, and energy @ break. The properties listed above are typically better than the combination of untreated crumb and new tire rubber mix control values when the combination of biologically treated crumb rubber and new tire rubber are analyzed and evaluated in Banbury tests.
In accordance with this invention, it has been demonstrated that particles of used crumb rubber can be reacted with active cultures of a bacillus-like bacterium isolated from a natural hot sulfur spring. The preferred isolate shows most favorable growth at 65xc2x0 C. and has been found to react with Sxe2x80x94S, and Sxe2x80x94C bonds and provide an improved reaction surface on the rubber. In particular, the bacterium interacts with the surface sulfur constituents and alters the surface chemistry and reactivity of the so treated vulcanized crumb tire rubber. It has been found that the biotreatment of the crumb rubber particle changes the surface reactivity of the crumb rubber particles to an extent that the rubber particles can be integrated into a new tire polymer mix at much higher levels than previously accomplished. The biotreatment of the crumb rubber provides a treated rubber product which is compatible with the new rubber polymer component of a tire mix.
The use of the biotreatment to modify select chemical species provides a process which can be carried out on. an economical basis. The process is thought to be particularly useful in that the biotreatment is believed to alter a wide variety of chemical additives used by tire manufacturers and which are present within the used rubber. The ability to alter these chemical constituents, further enhances the usefulness and compatibility of the treated used rubber with new polymer. The biological treatment provides a more standardized crumb rubber for use in recycling operations, including the incorporation of significant quantities and sizes of treated crumb rubber into polymer formulations for automobile and truck tires. Further, the biological organism is itself a renewable, regenerating resource.
The biological treatment process also facilitates the use of used tire rubber as a renewable resource. Accordingly, the biotreatment process enables the use of a heretofore difficult to recycle waste product into a component for high quality, high end cured rubber products.
The use of the biotreatment provides a process and product which can be operated on an economical basis at a commercial scale and does not result in the production of environmentally unacceptable waste products or emissions. Further, the elevated temperatures at which the preferred microorganisms operate can be maintained using waste heat which results from any of several steps in the commercial manufacturing of new rubber articles such as tires.
It is thus an object of this invention to provide a biotreatment process which increases by an effective amount the reactivity of used crumb rubber such that an increased amount of used crumb rubber can be mixed with a new tire polymer mix.
It is another object of this invention to provide a crumb rubber having an improved surface chemistry suitable for use in compounding a new tire rubber mix.
It is still another object of this invention to provide a treated crumb rubber product having a consistent surface chemistry in which polymer degrading constituents have been rendered inactive.
It is yet another object of this invention to provide a compounded rubber mix containing increased amounts of crumb rubber in the formulation.
It is still a further object of the invention to utilize waste heat generated from a rubber curing process to maintain a reactive culture at an elevated temperature such that the culture is kept substantially free of unwanted microbial contamination.
It is yet another object of this invention to provide a compounded rubber mix containing large sizes of a biologically treated rubber particulate.
It is a further object of this invention to provide a process of devulcanizing a rubber particulate in which the devulcanization is substantially limited to targeted chemical species and which leaves the physical properties of the particulate largely unchanged.
These and other objects of this invention are provided by a process for devulcanizing and/or otherwise conditioning or modifying particulate rubber comprising: providing a supply of crumb rubber, said crumb rubber having a plurality of Sxe2x80x94S and Cxe2x80x94S bonds; exposing the crumb rubber to microorganisms capable of using surface sulfur constituents as metabolic and/or energy sources, thereby oxidizing an effective amount of the Cxe2x80x94S and Sxe2x80x94S bonds; thereby, providing a crumb rubber having a treated surface defining an altered chemical state, the altered state being more reactive during formulation and curing into a new rubber product.
Reference now will be made in detail to the embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents. Other objects, features and aspects of the present invention are disclosed in or are obvious from the following detailed description. It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions.
A process for treating the crumb rubber is based upon the exposure of the crumb rubber to a selected microorganism which targets select chemical bonds. In particular, sulfur-sulfur (Sxe2x80x94S) and sulfur-carbon (Sxe2x80x94C) bonds on the crumb rubber surface are targeted. The biotreatment is believed to alter other constituents added to rubber, such as accelerators, antioxidants, and stabilizers. Alteration of these other constituents improves the quality of the treated crumb rubber product.
In the initial vulcanization of tire rubber, sulfur and sulfur containing compounds such as accelerators and initiators are added which crosslink with the tire polymer. The bonding and crosslinkage stabilize the polymer matrix, which imparts important desired properties to the rubber and increases the useful life of the tire. The presence of sulfur compounds within vulcanized tire rubber has limited the quantity of vulcanized rubber which may be used in new rubber mixes such as tires. The sulfur containing compounds and other additives present within the previously vulcanized rubber render a final product which is often brittle and has other properties which make it unsuitable for many uses and especially for use as a tire.
As used in the detailed description below, the term xe2x80x9csurface devulcanizedxe2x80x9d is used to indicate that certain surface properties of the particulate crumb rubber have been chemically altered such as by the application of specific microorganisms. It is believed that the surface activity of mono, di, and polysulfides which formed polymer cross linkages during the initial vulcanization process has been altered by the present biotreatment process. As such, the crumb rubber is referred to here as xe2x80x9csurface devulcanizedxe2x80x9d though it is understood that a substantial number of vulcanization products persist in the treated crumb rubber and in fact remain beneficial in the-overall reformulations of new rubber mixes with the treated crumb rubber. Again, it is to be understood that other chemical and/or physical changes may take place which have a beneficial effect on the used rubber when reformulated as part of a new rubber product or mixture.
The work herein uses a 40 mesh truck tread crumb rubber conforming to ASTM standard D5603. The crumb rubber was obtained from a commercial supplier of crumb rubber and is believed to represent a heterogenous mixture of different tire formulations from a variety of manufacturers. It is believed that the crumb rubber can be used over a wide range of particulate sizes, and the process is not size dependent solely for the size ranges used in the manufacturing of tires. For example, standard sizes of 200-40 mesh, tire buffings and even larger sizes are believed to be useful.
The ASTM standard crumb rubber material was chosen for its known uniformity. It is readily appreciated by those having ordinary skill in the art that crumb rubber originating from used passenger or truck tires will typically encompass products originating from numerous manufacturers and comprising an enormous assortment of chemical constituents. Accordingly, a wide variety of different chemicals are expected to be present on the surface of untreated crumb rubber. This is particularly true for the sulfur compounds originally added to the rubber during the vulcanization process. The present invention provides a treatment process which effectively modifies broad classes of reactive chemical constituents associated with the surface and adjacent subsurface of crumb rubber. Accordingly, the treatment process will address expected variations encountered in a commercial source of crumb rubber.
The examples below are intended to illustrate the present invention. The descriptions in no way limit the scope of the present invention.
In the examples, the properties of the combination of crumb and new tire rubber samples and appropriate controls are evaluated as follows:
Plasticity: Measurements carried out in accordance with ASTM Standard D1646.
Scorch: (t5, t35) Measurements carried out in accordance with ASTM Standard D1646.
Minimum Viscosity: Measurements carried out in accordance with ASTM Standard D1646.
Shore Hardness: Measurements carried out in accordance with ASTM Standard D2240.
Modulus of elongation at 300% and 100%: Measurements were carried out in accordance with ASTM Standard D412, test method A.
Tensile Strength: Measurements were carried out in accordance with ASTM Standard D412, test method A.
Elongation @ Break: Measured as a percentage value according to ASTM standard D412, test method A.
Energy @ Break: Measurements carried out in accordance with ASTM Standard D412, test method A as set forth in units of MPa.
Gxe2x80x2: Measurements carried out in accordance with ASTM Standard D2221.
Tan Delta: Hysteresis is expressed by the measurement of tan delta @ 10 percent deformation and at 23 degrees C in accordance with ASTM Standard D2231.
Analytical Characterization: The analytical protocols set forth in Table 1 and discussed further below are based upon standard ASTM measurements as set forth below.
Acetone extract measurements were made according to ASTM Standard D297-18, 19.
Oxygen measurements were made using commercially available oxygen analyzers.
Polymer ratios were determined according to ASTM Standard D3677.
Zinc oxide (%)xe2x80x94Measurements carried out as set forth in ASTM D297-47.
Macro ash measurements were made according to ASTM Standard D297-37.